Many optical processes of interest for various applications generally have improved performance as the input optical power increases. For example, the efficiency of second harmonic generation increases as the input power increases. Accordingly, methods for providing high optical power are of considerable interest. High optical power can be provided by a single high power source, or by effectively combining the outputs of two or more low power sources to provide a high power combined output. This second approach is generally referred to as optical power combining.
In most cases, it is desirable for the combined output radiation to be in a single spatial mode. Imposing the requirement of a single spatial mode combined output has significant consequences for optical power combining. In particular, interference between the optical inputs will occur in the combined single mode output unless the optical inputs are distinguishable (e.g., have different wavelengths and/or different states of polarization). In order to obtain power combining of interfering optical inputs, phase coherence of these optical inputs must be established, which typically requires implementation of an elaborate optical phase locking scheme. Accordingly, in applications that permit the use of distinguishable optical inputs, combination of distinguishable inputs is usually preferred. Such power combiners are often referred to as wavelength combiners, since radiation at different wavelengths is combined into a single spatial mode combined output. In this description, optical power combining is understood to include both wavelength combining and/or polarization combining.
A typical wavelength combiner has two or more input ports and a single output port, where each input port i has a corresponding wavelength acceptance range Δλi which is efficiently coupled to the output port. The wavelength ranges Δλi are substantially non-overlapping. One way to utilize such a wavelength combiner to combine the outputs of several lasers is to provide a set of lasers in one-to-one correspondence with the input ports, such that each laser has a fixed emission wavelength within the acceptance range Δλ for the corresponding input port. For example, such lasers could be semiconductor DFB lasers. While this approach is straightforward, it suffers from the disadvantage that providing lasers having emission wavelengths within the specified ranges can be costly in cases where the ranges are narrow. This cost issue is especially notable in cases where a large number of lasers are to be combined. Such wavelength combining by precisely selecting the wavelength of individual emitters is considered in U.S. Pat. No. 6,456,756.
An alternative approach for diode laser wavelength combining is to provide a set of diode lasers in one-to-one correspondence with the input ports, such that each laser has an emission wavelength which can be in any of the wavelength ranges Δλi. Since diode lasers tend to have a broad gain bandwidth, provision of such lasers is relatively straightforward (e.g., such lasers could be Fabry-Perot diode lasers). In this approach, a broadband partial reflector is optically coupled to the output port of the wavelength combiner. The combination of wavelength combiner and reflector provides wavelength-dependent feedback to each laser source. This linear feedback acts to set the emission wavelength of each laser source appropriately for wavelength combining. For example, a laser source coupled to a port j having an acceptance range Δλj will receive more feedback in the range Δλj than at other wavelengths, which will tend to force this source to lase at a wavelength within the range Δλj. With this approach, multiple diode lasers can be wavelength combined without the need for precise wavelength control of each laser diode. Thus less expensive laser diodes can be used. Such wavelength combining with linear feedback is considered in U.S. Pat. No. 6,567,580 and in U.S. Pat. No. 6,041,072.
However, certain problems which can arise in the context of wavelength combining are not addressed by the above approaches. An example of such a problem can arise in the context of wavelength combining to provide pump radiation for a parametric nonlinear optical process which is efficient over a relatively broad wavelength range. When either of the above approaches is used to provide wavelength combined pump radiation, the resulting pump radiation has a pump spectrum that is independent of the nonlinear optical process conversion efficiency. Since the pump spectrum remains fixed, careful and costly design of the broadband optical frequency converter can be required to obtain roughly constant conversion efficiency within the desired wavelength range.
Accordingly, it would be an advance in the art to provide wavelength combining for broadband optical frequency conversion that can automatically equalize conversion efficiency within a conversion wavelength range.